Add live wallpaper source folders and bundle preferences page icons

Live Wallpaper page gains a "Wallpaper Folders" list (mirroring the
static rotation one) plus an "Available Media" list of the videos/GIFs
found inside them to pick from, backed by a new live-wallpaper-folders
GSettings key and a generalized wallpaperSource.js extension scan.

The preferences page-switcher icons (General, Rotation, Live Wallpaper,
OLED Protection, About) were showing as broken-image placeholders since
they depended on exact symbolic icon names being present in the user's
system icon theme. Bundle a small set of custom SVG icons under icons/
and register them via Gtk.IconTheme.add_search_path so they always
resolve regardless of the system theme.

schemas/org.gnome.shell.extensions.benthicbloom.gschema.xml

@@ -79,6 +79,12 @@
       <description>Path to the video or animated GIF file used as the live wallpaper.</description>
     </key>
 
+    <key name="live-wallpaper-folders" type="as">
+      <default>[]</default>
+      <summary>Live wallpaper source folders</summary>
+      <description>List of folder paths scanned for videos and animated GIFs to pick a live wallpaper from.</description>
+    </key>
+
     <key name="live-wallpaper-muted" type="b">
       <default>true</default>
       <summary>Mute live wallpaper audio</summary>